Conventional electric water heaters have elongated heating elements comprising an outer tubular sheath enclosing an inner electrical resistance wire. In a typical element, the internal metallic resistance wire is surrounded by a material such as magnesium oxide which is an electrical insulator but is capable of a reasonably high heat transfer rate. The outer sheath may be formed of a metal such as copper or an INCOLOY material. Thermal energy passes from the hot resistance wire through the insulating material and sheath wall to the sheath surface, thereby heating the water.
Over time electric water heater elements tend to develop scale or calcium carbonate, which is a poor heat conductor. The heating element has a high heat flux so the poor thermal conductivity of the scale film tends to cause the heating element to overheat, which can lead to failure of the heating element. Also, the growth of scale on the element may physically deform the element and cause failure. Finally, as scale grows thick it tends to flake off from the element and into the heated water.
Various solutions have been proposed to alleviate the problems created by scaling of heating elements. For example, U.S. Pat. No. 5,586,214 to Eckman shows a water heater heating element which is alleged to minimize lime depositing. The Eckman heating element replaces the customary metallic sheath of the heating element with a plastic sheath. Attempts to coat heating elements with unconventional materials are usually unsuccessful due to adhesion problems or overheating.
In another proposed solution, the watt density is reduced so that scale will form at a lower rate, thus extending the element life. This may be accomplished by using a resistance wire of lower wattage rating, or increasing the sheath diameter and/or length. The disadvantages of this method are that an element of greater surface area is required, causing difficulties and fitting the element into smaller heater tanks, or increasing the cost through enlarged element size and enlarged port and mount size.
A scale-inhibiting water heater element suitable for use in conventional water heaters would be desirable.